Insight into the effect of N-doping level in N-doped carbons modified separator on improving the electrochemical performance of Li-S batteries

•Three nitrogen-doped carbons (NCs) with different nitrogen doping levels are prepared by a two-step pyrolysis procedure.•The results suggest that NC800 with appropriate N-doped content shows moderate adsorption of polysulfides as well as exhibits the best reaction kinetics on polysulfides conversion.•The NC800-PP assembled cells display high specific capacity and stable cycle capability. Carbon materials are widely applied in lithium-sulfur (Li-S) batteries due to their large specific surface area, high conductivity, and tunable properties. An appropriate heteroatom doping can enhance the adsorption ability and electrochemical activity toward lithium polysulfides, thus improving the overall performance of Li-S batteries. Herein, three nitrogen-doped carbons (NCs) with different nitrogen doping levels are firstly prepared and then used as the model materials to coat the common polypropylene (PP) separator, aiming to elaborate the effect of adsorption behavior from various N-doping level carbons surface toward lithium polysulfides on the subsequent catalytic conversion kinetics. The experimental data demonstrate that the NC800 with appropriate N-doped level shows the moderate adsorption of polysulfides as well as exhibits the best reaction kinetics on polysulfides conversion. In comparison, an assembled Li-S battery with NC800-PP separator delivers high capacity and good cycling stability, achieving an initial specific capacity of ∼ 1302.2 mAh g−1 at 0.2 C and a capacity fading rate of ∼ 0.061 % per cycle after 500 cycles at 1.0 C. Moreover, a Li-S battery using NC800-PP separator still delivers an initial specific capacity of ∼ 1147.6 mAh g−1 at 0.1 C and maintains a stable specific capacity of ∼ 833.5 mAh g−1 even with a sulfur loading of ∼ 3.9 mg cm−2. This study is believed to give better insight to reveal the internal correlation of adsorption and catalytic effect on polysulfides, as well as provide a guidance for rationally designing heteroatom doping carbon materials in Li-S batteries. [Display omitted]